1. Field of the Invention
This invention pertains to an overvoltage protector element in the form of a thyristor. More particularly, this invention pertains to a fail-safe device for use with such a thyristor.
2. Description of the Prior Art
In the telecommunications industry, the use of an overvoltage protection element is well known. An example of such a use is shown in commonly-assigned and copending U.S. Pat. No. 4,741,711.
In the aforementioned patent, a gas tube overvoltage protector element is employed. However, it is also known in the art to use a thyristor as an overvoltage protector element. A thyristor is a solid-state element which senses voltage. It is high-speed and may react to a lower voltage than that presently attainable with gas tubes. Also, the solid-state thyristor has a greater accuracy than the gas tubes.
The thyristor has tip and ring leads and a ground lead. The tip and ring leads are electrically connected to the ground lead in response to a sensed overvoltage condition. During periods of prolonged grounding, the thyristor achieves an elevated temperature.
A short period of grounding may result from a brief overvoltage condition. This could occur from lightening strikes. Prolonged grounding may result from prolonged overvoltage conditions. For example, a high voltage power line may fall and rest on a telephone line. This would result in a prolonged overvoltage condition on the telephone line.
Any grounding through the thyristor will increase the temperature of the thyristor. Prolonged grounding resulting from prolonged overvoltage conditions will result in significant temperature increases. Increased temperature present fire and other risks including personal injury exposure. To minimize these risks, prior art overvoltage protectors utilized so called fail-safe devices. Such devices would bypass the grounding of a thyristor and directly ground the tip or ring leads to the ground lead. As a result, extreme increases in temperature would be avoided.
The most common prior art fail-safe devices were commonly used with the aforementioned gas tube protector elements. Typically, such devices would include dielectric spacers which would space a grounding clip lead from contact areas on the protector element. In the event of elevated temperature, the dielectric spacer would deform or melt.
Within this application, a failed overvoltage protector element means any element which is experiencing elevated temperature resulting from prolonged overvoltage conditions. The term does not necessarily mean a faulty element.
It is an object of the present invention to provide a fail-safe device for a thyristor which has enhanced reliability and which is susceptible to low-cost manufacture and assembly.